The term “bioreactor” refers to an apparatus using a biological reaction caused by an enzyme or the like. Immobilized enzymes, immobilized microorganisms, other viable cells, and the like are used in biological reactions for bioreactors. Bioreactors are used for production of amino acids, peptides, organic acids, and other substances, isomerization of sugars, and the like. In addition, bioreactors comprising enzymatic catalysts are used in some processes in the petrochemical industry. Further, some other bioreactors are used for analysis for a variety of diagnostics and the like.
Biofuel cells are also referred to as enzymatic fuel cells in which electric energy is generated through chemical reactions caused by enzymes and microorganisms. Biofuel cells have a structure in which a cathode electrode and an anode electrode face each other across an electrolyte, as do general battery cells. Fuels used in biofuel cells are alcohols such as methanol and ethanol or sugars such as glucose. In addition, an immobilized enzyme is used in the cathode electrode and the anode electrode of an electrode biofuel cell.
Both bioreactors and biofuel cells are expected to result in significantly improved substance productivity and electric output if reaction efficiency of the chemical reaction caused by an enzyme or the like can be improved. However, research on technology that can improve enzymatic reactions caused by immobilized enzymes in bioreactors and biofuel cells has not been successful. Therefore, technology that can be universally applied to bioreactors and biofuel cells comprising immobilized enzymes in order to significantly improve the efficiency of enzymatic reactions caused by the immobilized enzymes has been awaited.
Non-Patent Document 1 and Patent Document 1 disclose biofuel cells in which an electron transfer mediator is immobilized on an electrode. Polyvinylimidazole is used as a solidifying agent for immobilizing an electron transfer mediator on an electrode material for the biofuel cells disclosed in Non-Patent Document 1 and Patent Document 1. In addition, in the cases of biofuel cells disclosed in Non-Patent Document 1 and Patent Document 1, enzymes are dispersed in an electrolyte solution.
In addition, Patent Document 2 discloses a fuel cell having a structure in which a positive electrode and a negative electrode face each other across an electrolyte containing a buffer substance, an immobilized enzyme is used for either one or both of the positive electrode and the negative electrode, and a compound containing an imidazole ring is used as the buffer substance. The fuel cell disclosed in Patent Document 2 is produced by immobilizing bilirubin oxidase on a positive electrode and, when an electrolyte containing an imidazole buffer solution is used, a high current density is achieved for unipolar evaluation of a cathode electrode.
Further, Patent Document 3 discloses a sensor for measuring a blood sugar level using an enzyme and an electron receptor. The sensor disclosed in Patent Document 3 includes a heterocyclic compound such as imidazole as well as an enzyme and an electron receptor so as to improve preservation stability of the enzyme (for suppression of changes in the current value before, during, and after preservation).